Teeth are very complex organs. To meet their mechanical requirements, vertebrate teeth are formed from two major distinct layers: an outer, hard, highly mineralized brittle layer of enamel, and a mineralized, but softer, less brittle layer of dentin. In a particular species, the pattern of dentition is fixed, but each tooth is formed independently of all others with a unique size, shape, and function. The relative location of each tooth is specified early in embryonic development by a distinct set of gene expressions, followed by other gene-regulated signals that specify the timing of tooth growth, and still other gene-regulated processes that determine the ultimate shape of the tooth. The cells in each layer are programmed to produce uniquely different proteinaceous structures that subsequently mineralize relatively late in tooth formation. Initial assembly of the tooth germ requires constant communication between the ectodermal cells of the oral epithelium, which differentiate to become ameloblasts and produce the enamel, and the underlying ectomesenchymal cells (also called mesenchymal cells), which become the dentin-producing odontoblasts. Ectomesenchymal cells are derived from the cells of the neural crest. Neural crest cell migrations that establish the oral cavity are rapid but highly controlled, and their cells have undergone several localized gene-regulated morphogenetic differentiation and commitment steps in reaching their positions. In the human fetus, the oral cavity is formed by week 4, with the cells of the oral ectoderm covering the connective tissue formed by the neural ectomesenchymal cells.

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